Sterile, disposable carbon dioxide and pH sensors of the types disclosed in the above-referenced patent applications are calibrated in vitro prior to use, but because of potential sensor drift, they must be recalibrated periodically during use to verify sensor readings. This is done in accordance with established practice by withdrawing a sample of the patient's blood by means of a syringe connected to the side arm of the catheter containing the sensor and measuring the electrochemical activity of the ion of interest using standard laboratory techniques. Alternatively, a blood sample may be obtained by a separate arterial puncture, or the sensor may be removed for in vitro calibration. There are significant disadvantages to each of these alternatives. They are time-consuming and require special laboratory services and, in addition, one requires multiple blood vessel penetrations while another presents the possibility of loss of sterility, sensor damage or clot formation within the catheter system.